Stream-in-air cell sorters are used in biomedical research and other fields to separate a mixture of cells or other small particles into various component types. They utilize a narrow stream of pressurized fluid (“sheath fluid”) that is ejected from a nozzle as a jet and which carries within it the cells or particles held in suspension. The stream flows through the path of one or more light sources, the light from which may be scattered by the cells and which may stimulate fluorescent molecules associated with the cells. At the end of the fluid jet (droplet formation region), the jet breaks up into predictable, regularly-spaced droplets which are individually given an electrical charge, and which are then precisely deflected by an applied electric field, so that droplets containing single desired cells are directed into a designated collection receptacle. Thus, a highly purified subset of cells or particles with specific characteristics that are identifiable by specific fluorescent markers can be obtained.
Accurate cell sorting requires well-controlled and precise fluid dynamics of the jet and droplet formation region. Adverse events, like a partial blockage of the nozzle discharge orifice, an air bubble in the fluid stream, or an accidental change in instrument settings, can disrupt the normal fluid dynamics of the stream (“stream failure”) and thereby interfere with proper sorting. This may result in a loss of desired cells and/or contamination of the collection receptacle(s) with unsorted cells, thereby ruining the purity of the sorted cells. To prevent or minimize this problem, the flow of the cell suspension sample and, if desirable, the flow of sheath fluid, must be interrupted as quickly as possible in the event of a stream failure. Some commercialized cell sorter models have automated this process as an integral part of their design, but many others have not. For the latter, the person operating the cell sorter must continuously monitor a live magnified video image of the stream and droplet formation region and be ready to react quickly to a stream failure by pressing a switch or the like to turn off the flow of cells and/or sheath fluid. The response of a human operator to a stream failure is limited by their ability to detect a change and by their reaction time, which may be exacerbated by inattention. Therefore, significant damage to the purity of the sorted cells may occur before a human operator can appropriately intervene. Also, continuous visual monitoring is a tedious and time-consuming task, since the operator must be constantly vigilant during the entire sorting time, often for many hours. Therefore, there is a need for a modular stream-monitoring and response system that can be utilized with steam-in-air sorters that do not incorporate such technology in their design.